Method and apparatus for generating gases for atomic absorption spectrophotometers

ABSTRACT

A gas generation accessory for an atomic absorption spectrophotometer includes a reaction vessel for holding a sample and an acid. The sample includes an element from a group including arsenic and selenium. A dosing column is coupled to the reaction vessel and a gas-tight stopcock is mounted in the dosing column and includes a cavity for holding a metal such as zinc. The stopcock is rotatable for projecting the metal into the acid to cause a reaction to release hydrogen gas. The hydrogen gas in turn reacts with the element in the sample to produce a gaseous hydride. The gaseous hydride is collected in an elastic reservoir and stored under pressure until the completion of the reaction. Thereupon the gaseous hydride in the elastic reservoir is carried into the atomic absorption spectrophotometer and the element appears in a substantial concentration for a relatively short time. This permits the resonance lines of the element to be recorded and the element detected.

United States Patent [191 Manning et al.

METHOD AND APPARATUS FOR GENERATING GASES FOR ATOMIC ABSORPTION SPECTROPHOTOMETERS [75] Inventors: David C. Manning, Trumbull; Frank J. Fernandez, Norwalk, both of Conn. I

[73] Assignee: The Perkin-Elmer Corporation,

Norwalk, Conn.

[22] Filed: Feb. 22, 1972 21 Appl. N6; 228,171

[52] US. Cl. 23/230 R, 23/253 R, 23/259, 356/36, 356/87 [51] Int. Cl. G0lj 3/30, GOln 1/00, GOln 21/58 [58] Field of Search 23/230 R, 253 R, 232 R,

OTHER PUBLICATIONS Menis et al., Anal. Chem. 41, N0. 7, June 1969,

i451 pr. 2,1974

Wiley et al., Anal. Chem. 4, No. 4, 1932, 396-397.

Primary Examiner-Robert M. Reese Attorney, Agent, or FirmDaniel R. Levinson [57] ABSTRACT A gas generation accessory for an atomic absorption spectrophotometer includes a reaction vessel for holding a sample and an acid. The sample includes an element from a group including arsenic and selenium. A dosing column is coupled to the reaction 'vessel and a gas-tight stopcock is mounted in the dosing column and includes a cavity for holding a metal such as zinc.

The stopcock is rotatable for projecting the metal into the acid to cause a reaction to release hydrogen gas. The hydrogen gas in turn reacts with the element in the sample to produce a gaseous hydride. The gaseous hydride is collected in an elastic reservoir and stored under pressure untilthe completion of the reaction. Thereupon the gaseous hydride in the elastic reservoir is carried into the atomic absorption spectrophotometer and the element appears in a substantial concentration for a relatively short time. This permits the resonance lines of the element to be recorded and the element detected.

FATENTEUAPR elem R 3.801.282

' sum 1 0f 2 I F 1 0 FUEL J6 sou/m5 56 RECORDER H FUEL 1% L SOURCE v METHOD AND APPARATUS FOR GENERATING GASES FOR ATOMIC ABSORPTION SPECTROPI-IOTOMETERS BACKGROUND OF THE INVENTION Certain elements, such as arsenic, present severe problems when the detection of such elements is attempted by standard atomic absorption sampling techniques. This is particularly true when samples contain the element in only very limited trace amounts. In conventional atomic absorption spectrophotometry a solution of the sample is nebulized to provide a vapor and the vapor is atomized in the flame of the spectrophotometer. Presently available nebulizers are only about percent efficient in forming droplets suitable for atomization by the atomic absorption flame. Furthermore the usual rate of aspirating a solution of 3 to 5 milliliters per minute for a length of time on the order of seconds causes the concentration to be extended over this length of aspiration. Consequently when an element such as arsenic appears in minute traces in the sample, the element tends to be indiscernible in the flame. Therefore there is at present a need for providing a technique for detecting elements such as arsenic in trace amounts in a sample.

SUMMARY OF THE INVENTION Agas generation accessory for an atomic absorption spectrophotometer includes a reaction vessel for holding an acid and a sample containing an element from a group consisting of arsenic and selenium. A dosing column is coupled to the reaction vessel to introduce a metal to release hydrogen gas from the acid to react with the element in the sample to form a gaseous hydride. An elastic gas reservoir is coupled to collect and to store under pressure the gaseous hydride as it is formed. At the completion of the reaction, the expansible reservoir releases the gaseous hydride to provide a surge of the element into the flame of the atomic absorption spectrophotometer to provide a substantial concentration of the element in a relatively short period of time.

DESCRIPTION OFTHE DRAWINGS FIG. I is a partially schematic and a partially isomet-' ric view of a gas generation accessory for an atomic absorption spectrophotometer;

FIG. 2 is a partially exploded isometric view of the accessory shown in FIG. 1;

FIGS. 3 and 4 are absorbance measurements of selected amounts of arsenic and selenium as measured by the apparatus of FIG. 1; and

FIGS. 5 and 6 are absorbance measurements of arsenic and selenium respectively to determine the 'minimum traces that are detectable in the apparatus of FIG. 1.

- DETAILED DESCRIPTION In FIG. 1 there is shown apparatus 10 for generating gases for analyzing by an atomic absorption spectrophotometer 12. The atomic absorption spectrophotometer 12 may for example comprise a Model 403 Atomic Absorption Spectrophotometer manufactured by The Perkin-Elmer Corporation of Norwalk, Connecticut. The spectrophotometer 12 is shown only in outline form in FIG. 1. The spectrophotometer 12 includes a burner and nebulizer mechanism 14 that nebulizes a 1 by means of a washer 63 and stop nut 61 fastened 1 a recorder 18. The recorder I8 may, for example, com-- prise a Model I65 Recorder manufactured by The Perkin-Elmer Corporation.

A gas generator accessory 20 is included in the apparatus 10 to extract in the form of a gaseous hydride an element from a group that includes arsenic and selenium. A sample that may for example contain a trace amount of arsenic is inserted into a reaction vessel or Erlenmeyer flask 22. The reaction vessel 22 may for example be supported by a jack 24 that is mounted on a stand 26. A dosing column 28 is inserted into the neck of the flask 22 and the dosing column 28 is supported on the stand 26 by means of a clamp 30 that is affixed to a support bar 31 rigidly mounted to the stand 26.

An inlet tube 32 and an outlet tube 34 are connected to and supported by the dosing column 28 for carrying the gaseous hydride created in the generator 20 to the burner nebulizer 14 in the spectrophotometer 12. Consequently the inlet tube 32 is coupled to an auxiliary source 36 of inert gas, such as argon. The inert gas is stored under pressure soas to carry the gaseous hydride out of the gas generator 20 and into'the outlet tube 34. The outlet tube 34 is coupled as diagrammatically indicated at 35 to the auxiliary oxidant tube 38 of the spectrophotometer 12. The inlet tube 32 and the outlet tube-34 support'a stopcock assembly 40. As shown more clearly in FIG. 2, the stopcock assembly 40 includes a rotatable stopcock 42 having a pair of apertures 44 and 46 penetrating through the stopcock 42 transverse to the axis thereof. The apertures 44 and 46 are positioned in the stopcock 42 such that the aperture 44 aligns 'with the outlet tube 34 and the aperture 46 aligns with the inlet tube 32 when the stopcock is in the position shown in FIG. 2. Thestopcock 40 is mounted in a stopcock assembly' housing 48 that is supported bythe inlet and outlet tubes 32 and 34. The stopcock 40 is held in the housing 48 by means of a washer 50 and stop nut 52 that fits the threaded end of the stopcock 40. The stopcock 40 includes a handle 54 for rotatingthe stopcock 40 within the housing 48. A bypass tube 56 interconnects the inlet tube 32 to the outlet tube 34 of the gas generator 20 when the stopcock 40 is rotated to close apertures 44 and 46. v

The closing column 28 also includes a closing stopcock assembly 60 which, includes a tapering stopcock 62 that contains a cavity 64 for storing a reagent therein. The cavity 64 is positioned'such that when the stopcock 62 is inserted into the dosing stopcock housing 65, the cavity 64 aligns itself with the dosing column 28. The dosing stopcock 62 is held in the housing to the threaded end of the'stopcock' 62.

An elastic gas reservoir is coupled to an apertured nipple 72 mounted in the dosing column 28. Nipple 72 includes a plurality of circumferential ridges 74 for firmly holding the neck 76 of theelastic reservoir 70. The elastic reservoir 70may for example comprise a resilient, expansible member that exhibits the capabilities of collecting and storing the gaseous hydride generated in generator 20. The reservoir then expels the gaseous hydride when the inlet-outlet stopcock 42 provides a path to the spectrophotometer 12. One elastic reservoir 70' utilized in practicing the invention was a latex balloon having a wall thickness of 0.012 inches.

Burner nebulizer 14 of the spectrophotometer 12 with hydrogen fuel derived from a source 80 coupled to the burner nebulizer 14 through a fuel line 82. To provide an argon-hydrogen entrained air flame in the spectrophotometer 12 an argon source 84 is coupled to the nebulizer tube 86 of the spectrophotometer l2.

OPERATION In preparing a sample for atomic absorption spectroscopy, approximately 20 milliliters of the sample is pipetted into the reaction vessel 22. The sample 23 should contain at least 001 micrograms of an element from the group including arsenic and selenium. In this description it is assumed that arsenic is the element contained in trace amounts in the sample. The next step in the gaseous hydride generation is the addition of concentrated hydrochloric acid of an amount sufficient to bring the sum of the sample and gas to 40 milliliters. .A l-milliliter volume of 20 percent stannous chloride solution is added to'the reaction vessel 22 and allowed to stand for several minutes after thorough mixing. The next step in the generation of the gaseous V hydride is the connection of the reaction vessel 22 to the dosing column 28. The support jack 24 may for example be lowered and raised such that there is easy insertion and removal of the reaction vessel 22 into the dosing column 28. Approximately 1.5 grams of meshed zinc metal is inserted into the open neck of the dosing column 28 for storage in the cavity 64 of the dosing stopcock 62.

The inlet-outlet stopcock 42 isthen rotated to connect the upper portion of the inlet tube 32 t the lower portion thereof as well as to connect the upper portion of the outlet tube 34 to the lower portion thereof. The

. auxiliary source 36 of argon therefore flushes out any oxygen contained in the reaction vessel 22 as well as in the rest of the system and causes this oxygen to be burned in the flame 16. Such oxygen would impede the formation of the gaseous hydride if allowed to remain in the system 10. The inlet-outlet stopcock 42 is then rotated to displace the apertures 44 and 46 from the inlet and outlet tubes 32 and 34 to effectively seal off The hydrogen released by this reaction then reacts with the arsenic in the sample to form arsine gas in accordance with the formula:

as well as free hydrogen gas.

The free hydrogen gas as well asthe arsine gaseous hydride created in the reaction vessel 22 flows into and this time, the inlet-outlet stopcock 42 is rotated to connect the gas generator 20 with the spectrophotometer 12. The auxiliary argon gas 36 therefore sweeps into the gas generator 20. The reduction in pressure in the gas generator 20 causes the elasticreservoir 70 to release the entrapped gaseous hydride and excess hydrogen gas and the argon stream flowing into the gas generato'r 20 from theargon source 36 sweeps the gases into the outlet tube 34. The gaseoushydride is therefore carried to the burner nebulizer l4 and atomized in the flame 16. A spectrophotometric examination is made of the element in the flame 16.

The extraction of minute traces of arsenic from the sample and their detection in the atomic absorption spectrophotometer 12 is made feasible by concentrating the arsine gas in the reservoir 70 and then releasing this concentration very rapidly into the burner nebulizer 14. In FIGS. 5 and 6 there are shown recordings of the detection in samples of 0.1 and 0.2 micrograms v of arsenic and selenium respectively, with the absorbance measured and recorded by'the recorder'lS. It is to be noted that such submicrogram traces of arsenic and selenium are detectable in the sample.

In FIGS. 3 and 4 there are shown the recorder tracings for solutions containing 0.5, l, 1.5 and 2 micrograms of arsenic and selenium respectively. It is to be noted that the gaseous hydrides generated produce increasingly discernible spikes in the output provided by the recorder 18. y

It appears that any element capable of forming a gaseous hydride such as antimony and bismuth is detectable in the gas generator 20 in addition to arsenic and selenium. Furthermore any element amenable to atomic absorption spectroscopy that can be converted to, any gaseous compound is analyzable by the apparams 10. Additionally the flame 16 is only one means of dissociating the metal hydride formed. Other means are a thermal furnace, an electrical furnace and the like.

Thus in accordance with the invention a gas generation accessory is provided for an atomic absorption spectrophotometer that permits samples containing trace amounts of particular elements that can be converted into gaseous compounds to bereadily'analyzed in the atomic absorption spectrophotometer. This is accomplished by extracting the elements from the sample to create the gaseous compound in an elastic reservoir and then expelling the stored gaseous cornpound into the flame of the spectrophotometer in a very short space of time. Such a projection produces a spiked output in the spectrophotometer which is readily discernible above the baseline fluctuation of the spectrophotometer. v

What is claimed is:

1. The method of analyzing in an atomic absorption spectrophotometer a sample containing an element from a group consisting of arsenic and selenium, comprising the steps of:

adding an acid to said sample in a reaction vessel,

purging said reaction vessel with an inert gas to remove oxygen from said vessel,

reacting a metal with said acid to release hydrogen gas,

closing off said reaction vessel during said reaction,

collecting in an elastic reservoir a gaseous hydride that is created by the reaction of said hydrogen gas with said element,

opening said reaction vessel at the end of said reaction so as to permit said elastic vessel to expel said gaseous hydride to said spectrophotometer to project a concentrated amount of said element in the form ofa gaseous hydride into said spectrophotometer in a short space of time.

2. The method as claimed in claim 1 wherein said acid comprises hydrochloric acid and said metal comprises zinc.

3. A gas generation accessory for an atomic absorption spectrophotometer comprising, in combination:

a reaction vessel adapted to contain an acid and a sample to be analyzed;

a dosing column coupled to said vessel for introducing a granularmetallic reagent into said vessel;

conduit includes an inlet tube for connection to a source of purging gas under pressure and an outlet tube for connection to the spectrophotometer.

5. An accessory, according to claim 4, including:

a bypass conduit interconnecting said inlet and outlet tubes and, shunted across the bypass conduit at a location between the bypass conduit and the reaction vessel, multi-position valve means adapted in one'position to permit flow through said inlet and outlet tubes into and out of said vessel and, in another position, to restrict flow in said inlet and outlet tubes to said bypass conduit. 

2. The method as claimed in claim 1 wherein said acid comprises hydrochloric acid and said metal comprises zinc.
 3. A gas generation accessory for an atomic absorption spectrophotometer comprising, in combination: a reaction vessel adapted to contain an acid and a sample to be analyzed; a dosing column coupled to said vessel for introducing a granular metallic reagent into Said vessel; an elastic expansible reservoir coupled in a gas-tight relation to said vessel for collection and storage under pressure of gaseous reaction products generated in the vessel; conduit means for conducting gaseous generation products from said vessel and reservoir to an atomic absorption spectrophotometer; and means for releasing such reaction products after generation thereof from said vessel and expansible reservoir to said conduit means.
 4. An accessory according to claim 3, wherein said conduit includes an inlet tube for connection to a source of purging gas under pressure and an outlet tube for connection to the spectrophotometer.
 5. An accessory, according to claim 4, including: a bypass conduit interconnecting said inlet and outlet tubes and, shunted across the bypass conduit at a location between the bypass conduit and the reaction vessel, multi-position valve means adapted in one position to permit flow through said inlet and outlet tubes into and out of said vessel and, in another position, to restrict flow in said inlet and outlet tubes to said bypass conduit. 